Separator and separator seal for polymer electrolyte fuel cells

ABSTRACT

A rubber composition comprising (A) an alkenyl-containing organopolysiloxane, (B) a silicone resinous copolymer, (C) an organohydrogenpolysiloxane, (D) fumed silica having a BET specific surface area of 50-400 m 2 /g, (E) carbon powder having a BET specific surface area of 30-150 m 2 /g, and (F) an addition reaction catalyst cures into a product which is useful as a separator seal in PEFCs. The rubber has a reduced compression set even in an acidic atmosphere or in contact with LLC, and the rubber itself has strength and good adhesion to separator substrates. The separator provides excellent seal performance over a long term.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2007-317420 filed in Japan on Dec. 7, 2007,the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to separators and separator seals for use inpolymer electrolyte fuel cells (PEFC) featuring compactness and moreparticularly, to separators and separator seals for PEFC which have along service life and ease of molding.

BACKGROUND ART

The fuel cell is capable of producing electricity without a substantialneed for fossil fuel that poses concerns about resource depletion,without noise, and at a high energy recovery rate as compared with otherenergy-based power generating systems. Great efforts have been made toexploit the fuel cell as a power generating plant of relatively compactsize in buildings and factories, with some cells having beencommercially implemented. In particular, polymer electrolyte fuel cells(PEFC) can operate at lower temperature than fuel cells of other types.The PEFC then draws attention not only as a device for householdco-generation, but also as the replacement power source for internalcombustion engines on vehicles because of the minimized corrosionconcern regarding the materials of which cell components are made andtheir ability to discharge relatively high current flow despite lowtemperature operation. The PEFC is constructed of electrolyte membranes,separators and other components. The separator is generally a platewhich is provided with a plurality of parallel channels on one surfaceor both surfaces. The separator plays the role of conducting theelectricity produced at the gas diffusion electrode within the fuel cellto the exterior, discharging water produced within the channels in thecourse of electricity generation, and securing the channels as a flowpath for incoming reaction gas to the fuel cell. Such a fuel cellseparator is required to be more compact in size. Since a multiplicityof separators are used in stack, there is a demand for a separator sealmaterial having durability and long term service.

As the separator sealing material, packing materials based on variousresins have been under study. Among them, sealing materials based onsilicone rubber are often used for their moldability, heat resistanceand elasticity. JP-A 11-129396 and JP-A 11-309747 disclose siliconerubber compositions of the addition cure type featuring easier moldingthan conventional silicone rubbers. Silicone rubbers obtained by curingthese compositions have been used, but are still unsatisfactory inmaintaining elasticity over a long term. In particular, these siliconerubbers are difficult to meet both acid resistance and seal performancein acidic aqueous solution which are requisite as the packing materialfor fuel cell separators. This problem can be solved by the use of asilicone resin as disclosed in JP-A 2002-309092. For the sealingmaterial for PEFC separators, not only acid resistance and lowcompression set, but also adhesion to the separator substrate areimportant factors. JP-A 2004-14150 describes a silicone rubber havingminimal compression set in long-life coolant (LLC) as an exemplarygasket material for carbon separators, but refers nowhere to theadhesion of the silicone rubber. JP-A 2007-146147 describes a primer tobe applied prior to integral molding of silicone rubber to a substrateto form a fuel cell separator. The rubber material is only describedtherein as being of the addition cure type, with no reference to itsdetail.

SUMMARY OF THE INVENTION

An object of the invention is to provide a separator seal for use inpolymer electrolyte fuel cells, which is made of cured rubber havingreduced compression set, improved acid resistance and good adhesion toseparator substrates; and a separator having the seal formed at theperiphery of a separator substrate.

The inventors have found that a rubber composition comprising (A) anorganopolysiloxane containing at least two silicon-bonded alkenyl groupsin a molecule, (B) a silicone resinous copolymer, (C) anorganohydrogenpolysiloxane containing at least three silicon-bondedhydrogen atoms in a molecule, (D) fumed silica having a BET specificsurface area of 50 to 400 m²/g, (E) carbon powder having a BET specificsurface area of 30 to 150 m²/g, and (F) an addition reaction catalystcures into a product which can be used as a separator seal, and that theresultant separator seal exerts excellent seal performance over a longterm because the rubber has a reduced compression set even in an acidicatmosphere created by an electrolyte membrane or in contact with LLC,and the rubber itself has acid resistance and good adhesion to separatorsubstrates.

Accordingly, in one aspect, the present invention provides a separatorseal for use in polymer electrolyte fuel cells which is formed of asealing composition in the cured state, said sealing compositioncomprising

(A) 100 parts by weight of an organopolysiloxane containing at least twoalkenyl groups each attached to a silicon atom in a molecule,

(B) 5 to 50 parts by weight of a resinous copolymer composed mainly ofR₃SiO_(1/2) units and SiO₂ units in a molar ratio between 0.5:1 and1.5:1, wherein R is a substituted or unsubstituted monovalenthydrocarbon group, and containing 5×10⁻³ to 1×10⁻⁴ mol/g of vinylgroups,

(C) 0.5 to 20 parts by weight of an organohydrogenpolysiloxanecontaining at least three hydrogen atoms each attached to a silicon atomin a molecule,

(D) 10 to 30 parts by weight of fumed silica having a BET specificsurface area of 50 to 400 m²/g,

(E) 0.1 to 3 parts by weight of carbon powder having a BET specificsurface area of 30 to 150 m²/g, and

(F) a catalytic amount of an addition reaction catalyst.

In preferred embodiments, the organopolysiloxane (A) has an averagedegree of polymerization of 100 to 2,000, at least 90 mol % of theentire organic groups attached to silicon atoms being methyl; a molarratio of Si—H groups in component (C) to total alkenyl groups incomponents (A) and (B), [Si—H/alkenyl], is between 0.8 and 5.0; theresinous copolymer (B) has a weight average molecular weight of 1,000 to10,000.

In another aspect, the present invention provides a separator for use inPEFCs comprising a substrate comprising a metal thin plate or aconductive powder and a binder and a seal formed at a periphery on atleast one surface of the substrate, the seal comprising a cured productof the sealing composition.

BENEFITS OF THE INVENTION

The sealing composition of the invention cures into a rubber productwhich undergoes a reduced compression set even in an acidic atmospherecreated by an electrolyte membrane or in contact with LLC, and which hasa high strength in itself, ease of molding and good adhesion toseparator substrates so that it exerts excellent seal performance over along term. It is best suited as a separator seal for use in PEFCs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective exploded view showing main components of apower-generating cell in a fuel cell stack according to one embodimentof the invention.

FIG. 2 is a cross-sectional view of the fuel cell stack taken alonglines II-II in FIG. 1.

FIG. 3 is a cross-sectional view traversing a fuel gas inletcommunication hole of the fuel cell stack.

FIG. 4 is a cross-sectional view traversing an oxidant gas inletcommunication hole of the fuel cell stack.

FIG. 5 is a front view of a first metal separator constituting thepower-generating cell.

FIG. 6 is a front view showing one surface of a second metal separatorconstituting the power-generating cell.

FIG. 7 is a front view showing the other surface of the second metalseparator constituting the power-generating cell.

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The sealing material or sealing rubber composition of the invention isused to form a seal at a periphery on at least one surface of aseparator for use in polymer electrolyte fuel cells (PEFCs). The sealingcomposition comprises as essential components,

(A) 100 parts by weight of an organopolysiloxane containing at least twoalkenyl groups each attached to a silicon atom in a molecule,

(B) 5 to 50 parts by weight of a resinous copolymer composed mainly ofR₃SiO_(1/2) units and SiO₂ units in a molar ratio [R₃SiO_(1/2)/SiO₂]between 0.5:1 and 1.5:1, wherein R is a substituted or unsubstitutedmonovalent hydrocarbon group, and containing 5×10⁻³ to 1×10⁻⁴ mol/g ofvinyl groups,

(C) 0.5 to 20 parts by weight of an organohydrogenpolysiloxanecontaining at least three hydrogen atoms each attached to a silicon atomin a molecule,

(D) 10 to 30 parts by weight of fumed silica having a BET specificsurface area of 50 to 400 m²/g,

(E) 0.1 to 3 parts by weight of carbon powder having a BET specificsurface area of 30 to 150 m²/g, and

(F) a catalytic amount of an addition reaction catalyst.

Component A

Component (A) is an organopolysiloxane containing at least two alkenylgroups each attached to a silicon atom in a molecule. Most often, it isrepresented by the following average compositional formula (I):R¹ _(a)SiO_((4-a)/2)  (I)wherein R¹ which may be the same or different is a is substituted orunsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms,preferably 1 to 8 carbon atoms, and “a” is a positive number in therange of 1.5 to 2.8, preferably 1.8 to 2.5.

Examples of the substituted or unsubstituted monovalent hydrocarbongroup represented by R¹ include alkyl groups such as methyl, ethyl,propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl,hexyl, cyclohexyl, octyl, nonyl, and decyl; aryl groups such as phenyl,tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyland phenylpropyl; alkenyl groups such as vinyl, allyl, propenyl,isopropenyl, butenyl, hexenyl, cyclohexenyl and octenyl, as well assubstituted forms of the foregoing groups in which some or all hydrogenatoms are replaced by halogen atoms (e.g., fluoro, bromo and chloro),cyano groups or the like, such as chloromethyl, chloropropyl,bromoethyl, trifluoropropyl and cyanoethyl. Preferably, at least 90 mol% of the entire R¹ groups are methyl.

At least two of R¹ should be alkenyl groups, preferably of 2 to 8 carbonatoms, more preferably 2 to 6 carbon atoms, and most preferably vinyl.The content of alkenyl groups is preferably 5.0×10⁻⁶ mol/g to 5.0×10⁻³mol/g, more preferably 1.0×10⁻⁵ mol/g to 1.0×10⁻³ mol/g of theorganopolysiloxane. An alkenyl content of less than 5.0×10⁻⁶ mol/g maygive too low a rubber strength to provide a satisfactory seal whereas analkenyl content of more than 5.0×10⁻³ mol/g may result in a highercrosslinked density and hence, brittle rubber.

The alkenyl groups may be attached to a silicon atom at the end of themolecular chain or a silicon atom midway the molecular chain or both.The inclusion of at least alkenyl groups attached to silicon atoms atboth ends of the molecular chain is preferred.

The preferred organopolysiloxane basically has a straight chainstructure, but may partially have a branched or cyclic structure. Themolecular weight is not particularly limited and the useful siloxane mayrange from a low viscosity liquid to a high viscosity gum. Theorganopolysiloxane should preferably have an average degree ofpolymerization of 100 to 2,000, more preferably 150 to 1,500. An averagedegree of polymerization of less than 100 may lead to a cured siliconerubber with insufficient elasticity to achieve a seal whereas an averagedegree above 2,000 may lead to a silicone rubber composition which istoo viscous to mold. It is noted that the degree of polymerization is aweight average value as determined by gel permeation chromatography(GPC) versus polystyrene standards.

Component B

Component (B) is a resinous copolymer composed mainly of R₃SiO_(1/2)units and SiO₂ units. R is selected from substituted or unsubstitutedmonovalent hydrocarbon groups, preferably having 1 to 10 carbon atoms,more preferably 1 to 8 carbon atoms. Examples of the monovalenthydrocarbon group represented by R include alkyl groups such as methyl,ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl,neopentyl, hexyl, cyclohexyl, octyl, nonyl, and decyl; aryl groups suchas phenyl, tolyl, xylyl and naphthyl; aralkyl groups such as benzyl,phenylethyl and phenylpropyl; alkenyl groups such as vinyl, allyl,propenyl, isopropenyl, butenyl, hexenyl, cyclohexenyl and octenyl, aswell as substituted forms of the foregoing groups in which some or allhydrogen atoms are replaced by halogen atoms (e.g., fluoro, bromo andchloro), cyano groups or the like, such as chloromethyl, chloropropyl,bromoethyl, trifluoropropyl and cyanoethyl.

The resinous copolymer may consist of R₃SiO_(1/2) units and SiO₂ unitsor if desired, additionally contain R₂SiO_(2/2) units and/or RSiO_(3/2)units (wherein R is as defined above) in a total amount of up to 50%,preferably up to 40%, based on the total weight of the copolymer. Themolar ratio of R₃SiO_(1/2) units to SiO₂ units, [R₃SiO_(1/2)/SiO₂]should be from 0.5:1 to 1.5:1, preferably from 0.5:1 to 1.3:1. A molarratio of less than 0.5 leads to less acid resistance whereas a molarratio of more than 1.5 leads to increased compression set andcompromises the compatibility with component (A) to impede blending.

The resinous copolymer (B) should contain 5×10⁻³ to 1×10⁻⁴ mol/g, andpreferably 3×10⁻³ to 2×10⁻⁴ mol/g of vinyl groups. A vinyl content ofmore than 5×10⁻³ mol/g results in a hard brittle rubber which achievesan insufficient seal whereas a vinyl content of less than 1×10⁻⁴ mol/gleads to poor acid resistance.

It is preferred for improved acid resistance that the resinous copolymerhave a weight average molecular weight (Mw) of 1,000 to 10,000, and morepreferably 2,000 to 8,500, as measured by GPC versus polystyrenestandards. A Mw of less than 1,000 may fail to provide acid resistancewhereas a copolymer with a Mw in excess of 10,000 may be awkward toproduce or to incorporate in rubber compositions.

The resinous copolymer can generally be prepared by hydrolyzing asuitable chlorosilane and alkoxysilane in a well-known manner.

The amount of the resinous copolymer (B) blended is 5 to 50 parts,preferably 5 to 40 parts by weight per 100 parts by weight of component(A). Less than 5 parts of the resinous copolymer fails to providesatisfactory acid resistance whereas more than 50 parts leads toincreased compression set.

Component C

Component (C) is an organohydrogenpolysiloxane containing at least threehydrogen atoms each attached to a silicon atom (i.e., Si—H groups) in amolecule. It serves as a crosslinking agent for curing the compositionwherein Si—H groups in the molecule crosslink with silicon-bondedalkenyl groups in components (A) and (B) through hydrosilylationaddition reaction.

Most often, the organohydrogenpolysiloxane (C) is represented by thefollowing average compositional formula (II):R² _(b)H_(c)SiO_((4-b-c)/2)  (II)wherein R² is each independently a substituted or unsubstitutedmonovalent hydrocarbon group having 1 to 10 carbon atoms, preferably 1to 8 carbon atoms, “b” is a positive number of 0.7 to 2.1, “c” is apositive number of 0.001 to 1.0, and b+c is from 0.8 to 3.0. Preferredare those of formula (II) having at least three (typically 3 to 300),more preferably 3 to 100, most preferably 3 to 50 silicon-bondedhydrogen atoms in a molecule.

Examples of the monovalent hydrocarbon group represented by R² are asexemplified above for R¹ although groups free of aliphatic unsaturation(as in alkenyl groups) are preferred. Preferably, “b” is 0.8 to 2.0, “c”is 0.01 to 1.0, and b+c is from 1.0 to 2.5.

The molecular structure of the organohydrogenpolysiloxane may bestraight, cyclic, branched or three-dimensional network. The number ofsilicon atoms per molecule or the degree of polymerization is preferablyabout 2 to about 300, especially about 4 to about 150. Differentlystated, the preferred organohydrogenpolysiloxanes are those which areliquid at room temperature (25° C.) and specifically have a viscosity ofup to 1,000 mPa-s, and more preferably 0.1 to 500 mPa-s at 25° C. asmeasured by a rotational viscometer.

Notably, the silicon-bonded hydrogen atom may be positioned at the endor an intermediate of the molecular chain or both.

Exemplary of the organohydrogenpolysiloxane (C) aretrimethylsiloxy-endcapped methylhydrogenpolysiloxane,trimethylsiloxy-endcapped dimethylsiloxane-methylhydrogensiloxanecopolymers, dimethylhydrogensiloxy-endcapped dimethylpolysiloxane,dimethylhydrogensiloxy-endcapped dimethylsiloxane-methylhydrogensiloxanecopolymers, trimethylsiloxy-endcappedmethylhydrogensiloxane-diphenylsiloxane copolymers,trimethylsiloxy-endcappedmethylhydrogensiloxane-diphenylsiloxane-dimethylsiloxane copolymers,copolymers composed of (CH₃)₂HSiO_(1/2) units and SiO_(4/2) units, andcopolymers composed of (CH₃)₂HSiO_(1/2) units, SiO_(4/2) units and(C₆H₅)SiO_(3/2) units. As used herein, the term “endcapped” means thatthe polysiloxane is capped at both ends of its molecular chain with theindicated groups.

The amount of the organohydrogenpolysiloxane (C) blended is 0.5 to 20parts, and preferably 0.6 to 15 parts by weight, per 100 parts by weightof component (A). The molar ratio of silicon-bonded hydrogen atoms (Si—Hgroups) in component (C) to the total of alkenyl groups in components(A) and (B), [Si—H/alkenyl], is preferably from 0.8:1 to 5.0:1,especially from 1.0:1 to 3.0:1. A molar ratio outside this range maylead to cured rubber with increased compression set, aggravating theseal performance.

Component D

Component (D) is fumed silica which is essential to impart satisfactorystrength to silicone rubber. The fumed silica should have a specificsurface area of 50 to 400 m²/g, and preferably 100 to 350 m²/g, asmeasured by the BET method. A surface area below 50 m²/g may compromiseacid resistance whereas above 400 m²/g, compression set increases. Thefumed silica may be used as such, but preferably after treatment with asurface hydrophobizing agent. Alternatively, a surface treating agent isadded when the fumed silica is mixed with the silicone fluid, wherebythe fumed silica is treated during the mixing step. Suitable surfacetreating agents are well-known agents including alkylalkoxysilanes,alkylchlorosilanes, alkylsilazanes, silane coupling agents, titanatetreating agents, and fatty acid esters alone or in admixture. When twoor more agents are used, they may be applied at the same time ordifferent times.

The amount of fumed silica (D) blended is 10 to 30 parts, especially 12to 28 parts by weight, per 100 parts by weight of component (A). Lessthan 10 parts of the fumed silica fails to provide satisfactory rubberstrength whereas more than 30 parts increases compression set,aggravating the seal performance.

Component E

Component (E) is carbon powder and essential for improving the adhesionto separator substrates and the compression set of the composition.Carbon black is typical of the carbon powder. Examples of suitablecarbon black include acetylene black, conductive furnace black (CF),super-conductive furnace black (SCF), extra-conductive furnace black(XCF), conductive channel black (CC), and furnace black and channelblack which have been heat treated at high temperatures of the order of1500° C.

Specific examples include acetylene blacks sold under the trade name ofDenka Acetylene Black from Denki Kagaku Kogyo K.K. and ShawniganAcetylene Black from Shawnigan Chemical Co.; conductive furnace blackssold under the trade name of Continex CF from Continental Carbon andVulcan C from Cabot Corp.; super-conductive furnace blacks sold underthe trade name of Continex SCF from Continental Carbon and Vulcan SCfrom Cabot Corp.; extra-conductive furnace blacks sold under the tradename of Asahi HS-500 from Asahi Carbon Co., Ltd. and Vulcan XC-72 fromCabot Corp.; and conductive channel black sold under the trade name ofCorax L from Degussa AG. Ketjen Black EC and Ketjen Black EC-600JD(Ketjen Black International) which belong to a class of furnace blackare also useful.

The carbon powder should have a specific surface area of 30 to 150 m²/gas measured by the BET method. Preferably the surface area is 35 to 120m²/g and more preferably 40 to 100 m²/g. A surface area below 30 m²/gmay fail to provide adhesion whereas a surface area in excess of 150m²/g has a deleterious impact on the compression set. Further the carbonpowder has an iodine adsorption value of 30 to 120 mg/g, and preferably40 to 100 mg/g, and a DBP absorption value of 100 to 200 ml/100 g, andpreferably 120 to 180 ml/100 g. Outside the ranges, carbon powder maybecome ineffective in facilitating adhesion and give deleterious impactsto compression set and the like. It is noted that the iodine adsorptionand DBP absorption values are measured according to JIS K1474 and JISK6221, respectively.

The amount of carbon black blended is 0.1 to 3 parts, especially 0.3 to2 parts by weight, per 100 parts by weight of component (A). Less than0.1 part of carbon black fails to provide satisfactory adhesion whereasmore than 3 parts has a deleterious impact on compression set andadversely affects insulation (i.e., allows the composition to beelectrically conductive). Since the seal formed from the compositionherein should be not only a seal against air, hydrogen gas and coolingmedia (e.g., LLC), but also an insulating seal, carbon black ispreferably blended in such amounts that the cured rubber may have aresistivity (volume resistivity) equal to or more than 1.0 TΩ-m.

Component F

Component (F) is an addition reaction catalyst for promoting additionreaction between alkenyl groups in the organopolysiloxanes as components(A) and (B) and silicon-bonded hydrogen atoms (Si—H groups) in theorganohydrogenpolysiloxane as component (C). Most often, the catalyst isselected from platinum group metal-based catalysts including platinumcatalysts such as platinum black, platinic chloride, chloroplatinicacid, reaction products of chloroplatinic acid with monohydric alcohols,complexes of chloroplatinic acid with olefins, and platinumbisacetoacetate as well as palladium catalysts and rhodium catalysts,with the platinum catalysts being preferred.

The amount of the catalyst blended is a catalytic amount to promoteaddition reaction and usually about 0.5 to 1,000 ppm, especially about 1to 500 ppm of platinum group metal based on the weight of component (A).Less than 0.5 ppm may be ineffective to promote addition reaction,leading to undercure. Amounts of more than 1,000 ppm may exert littlefurther effect on the addition reaction and be uneconomical.

Other Components

If necessary, the composition may further contain other components, forexample, fillers such as precipitated silica, ground quartz,diatomaceous earth and calcium carbonate; hydrosilylation reactionregulating agents such as nitrogen-containing compounds, acetylenecompounds, phosphorus compounds, nitrile compounds, carboxylates, tincompounds, mercury compounds, and sulfur compounds; heat resistanceimprovers such as iron oxide and cerium oxide; internal parting agentssuch as dimethylsilicone fluid; tackifiers, and thixotropic agents.

Separator Seal

The separator seal is formed of the sealing material or additionreaction cure type silicone rubber composition comprising theabove-described components in the cured state. The silicone rubbercomposition may be applied and cured by well-known techniques, forming aseal on a PEFC separator.

More particularly, when PEFC separator seals are manufactured using thecured rubber, the silicone rubber composition is molded into a sealshape by a compression molding, casting or injection molding technique,and the molded seal is combined with a separator substrate.Alternatively, integrated seal-separator substrate members aremanufactured by dipping, coating, screen printing, or insert molding.Preferred curing conditions for the silicone rubber composition includea temperature of 100 to 300° C. and a time of 10 seconds to 30 minutes.

The separator substrate used herein may be a metal thin plate or asubstrate manufactured by integral molding of an electrically conductivepowder in a binder. A seal is formed from the silicone rubbercomposition along the periphery of this separator substrate by theabove-described technique, whereupon a PEFC separator within the scopeof the invention is available.

Examples of the conductive powder include natural graphite such as flakegraphite, artificial graphite, and conductive carbon blacks such asacetylene black and Ketjen Black. Any powders may be used as long asthey are electrically conductive. Suitable binders include epoxy resins,phenolic resins, and rubber-modified phenolic resins.

According to the invention, the silicone rubber composition is appliedand cured to the periphery of a separator substrate by a suitabletechnique such as compression molding, casting, injection molding,transfer molding, dipping, coating or screen printing. Thus the curedsilicone rubber composition forms a seal. This results in a separatorfor PEFCs in which a ring-like seal (separator seal) is formed on thesubstrate so as to circumferentially extend along the periphery of thesubstrate.

Preferably the seal has a thickness or height of 0.1 to 2 mm. A seal ofless than 0.1 mm may be difficult to form and exert less sealing effectswhereas a seal of more than 2 mm may be inconvenient for size reduction.

Now referring to the figures, some embodiments of the separator forPEFCs according to the invention are illustrated. The invention is notlimited thereto.

FIG. 1 is a perspective exploded view showing main components of apower-generating cell 12 constituting a fuel cell stack 10 according toone embodiment of the invention. A plurality of power-generating cells12 are stacked in the direction of arrow A to construct the fuel cellstack 10. FIG. 2 is a cross-sectional view of this fuel cell stack 10taken along lines II-II in FIG. 1.

As shown in FIGS. 2 to 4, the fuel cell stack 10 includes a plurality ofpower-generating cells 12 stacked in the direction of arrow A and endplates 14 a, 14 b at opposite ends in the stacking direction. The endplates 14 a, 14 b are fixedly tied via tie rods (not shown) so that afastening load is applied across the stacked cells 12 in the directionof arrow A.

As shown in FIG. 1, each power-generating cell 12 includes anelectrolyte membrane-electrode assembly (MEA) 16 interposed betweenfirst and second metal separators 18 and 20. The first and second metalseparators 18 and 20 are, for example, steel plates, stainless steelplates, aluminum plates, plated steel plates or such metal plates whichhave been surface treated to be corrosion resistant. Their thickness isset in the range of 0.05 to 1.0 mm, for example.

At one side edge of the power-generating cell 12 in the direction ofarrow B (in FIG. 1, typically horizontal direction), an oxidant gasinlet communication hole 30 a for feeding an oxidant gas such asoxygen-containing gas, a coolant outlet communication hole 32 b fordischarging a coolant medium, and a fuel gas outlet communication hole34 b for discharging a fuel gas such as hydrogen-containing gas, whichare in fluid communication with corresponding holes in adjacent cells inthe direction of arrow A or stacking direction, are arranged in thedirection of arrow C (typically vertical direction).

At the other side edge of the power-generating cell 12 in the directionof arrow B, a fuel gas inlet communication hole 34 a for feeding thefuel gas, a coolant inlet communication hole 32 a for feeding thecoolant medium, and an oxidant gas outlet communication hole 30 b fordischarging the oxidant gas, which are in fluid communication withcorresponding holes in adjacent cells in the direction of arrow A, arearranged in the direction of arrow C.

Specifically, the MEA 16 includes a solid polymer electrolyte membrane36 in the form of a perfluorocarbon sulfonic acid thin film impregnatedwith water, which is sandwiched between an anode (or first electrode) 38and a cathode (or second electrode) 40. The anode 38 has a smallersurface area than the cathode 40.

The anode 38 and cathode 40 each include a gas diffusion layer formed ofcarbon paper or the like, and an electrocatalytic layer which is formedby uniformly applying porous carbon particles having a platinum alloysupported on their surfaces to the surface of the gas diffusion layer.The electrocatalytic layers are joined to the opposite surfaces of thesolid polymer electrolyte membrane 36.

The first and second metal separators 18 and 20 have inner surfaces 18 aand 20 a facing MEA 16 and outer surfaces 18 b and 20 b, respectively.The inner surface 18 a of first metal separator 18 is provided withoxidant gas flow channels (reaction gas flow channels) 42 which extendin a serpentine manner in the direction of arrow B and vertically upward(see FIGS. 1 and 5). As shown in FIG. 6, the inner surface 20 a ofsecond metal separator 20 is provided with fuel gas flow channels(reaction gas flow channels) 44 which are in fluid communication withfuel gas inlet communication hole 34 a and fuel gas outlet communicationhole 34 b as will be described later, and extend in a serpentine mannerin the direction of arrow B and vertically upward (in the direction ofarrow C).

As shown in FIGS. 1 and 2, coolant flow channels 46 are defined betweenthe surfaces 18 b and 20 b of first and second metal separators 18 and20 and in fluid communication with coolant inlet and outletcommunication holes 32 a and 32 b. The coolant flow channels 46 extendstraight in the direction of arrow B.

As shown in FIGS. 1 to 5, a first seal member 50 extendscircumferentially along the peripheral edge of first metal separator 18and is integrally joined to the surfaces 18 a and 18 b of first metalseparator 18. The first seal member 50 is formed by applying the rubbercomposition to the separator substrate by a technique such ascompression molding, casting, injection molding, transfer molding,dipping, coating or screen printing, and curing.

The first seal member 50 includes a first planar portion 52 which isintegrally joined to the surface 18 a of first metal separator 18 and asecond planar portion 54 which is integrally joined to the surface 18 bof first metal separator 18. The second planar portion 54 extends longerthan the first planar portion 52.

As shown in FIGS. 2 and 3, the first planar portion 52 extendscircumferentially at a position outward spaced apart from the peripheraledge of MEA 16, and the second planar portion 54 extendscircumferentially over a region overlying a certain portion of cathode40. As shown in FIG. 5, the first planar portion 52 is formed such thatoxidant gas inlet and outlet communication holes 30 a and 30 b are influid communication with oxidant gas flow channels 42, and the secondplanar portion 54 is formed such that coolant inlet communication hole32 a is in fluid communication with coolant outlet communication hole 32b.

A second seal member 56 extends circumferentially along the peripheraledge of second metal separator 20 and is integrally joined to thesurfaces 20 a and 20 b of second metal separator 20. On the surface 20 aside of second metal separator 20, the second seal member 56 is providedwith an outside seal 58 a which is disposed on surface 20 a in proximityto the peripheral edge of second metal separator 20, and an inside seal58 b which is inwardly spaced apart from the outside seal 58 a. Theoutside and inside seals 58 a and 58 b are provided on one side ofsecond seal member 56 facing the anode 38.

The outside and inside seals 58 a and 58 b may have any desired shapeselected from a tapered (or lip), trapezoid and semicylindrical shape.The outside seal 58 a is in contact with first planar portion 52 offirst metal separator 18, and the inside seal 58 b is in direct contactwith solid polymer electrolyte membrane 36 of MEA 16.

As shown in FIG. 6, the outside seal 58 a circumscribes oxidant gasinlet communication hole 30 a, coolant outlet communication hole 32 b,fuel gas outlet communication hole 34 b, fuel gas inlet communicationhole 34 a, coolant inlet communication hole 32 a and oxidant gas outletcommunication hole 30 b. The inside seal 58 b circumscribes fuel gasflow channels 44. The peripheral edge of MEA 16 is disposed betweenoutside and inside seals 58 a and 58 b.

On the surface 20 b side of second metal separator 20, the second sealmember 56 is provided with an outside seal (coolant seal) 58 c whichcorresponds to outside seal 58 a, and an inside seal 58 d whichcorresponds to inside seal 58 b (see FIG. 7). The outside and insideseals 58 c and 58 d have the same shape as outside and inside seals 58 aand 58 b.

As shown in FIG. 6, the outside seal 58 a is provided with an inletmanifold 60 which establishes fluid communication between oxidant gasinlet communication hole 30 a and oxidant gas flow channels 42, and anoutlet manifold 62 which establishes fluid communication between oxidantgas outlet communication hole 30 b and oxidant gas flow channels 42.

The inlet manifold 60 is constructed by a plurality of supports 64 whichare formed by cutting off outside seal 58 a at positions spaced apart inthe direction of arrow C and extend in the direction of arrow B.Communication paths for oxidant gas are defined between supports 64. Theoutlet manifold 62 is similarly constructed by a plurality of supports66 which are formed by partially cutting off outside seal 58 a andextend in the direction of arrow B. The supports 66 are in contact withfirst planar portion 52 to define communication paths for oxidant gastherebetween.

The supports 64 of inlet manifold 60 overlie seal laps 68 of outsideseal 58 c while being on the opposite surfaces 20 a, 20 b of secondmetal separator 20. Notably, the seal laps 68 are portions of outsideseal 58 c that overlie supports 64 of outside seal 58 a, with secondmetal separator 20 interposed therebetween.

The outlet manifold 62 is constructed as is the inlet manifold 60. Thesupports 64 and seal laps 70 of outside seal 58 c that overlie eachother on the opposite surfaces 20 a, 20 b of second metal separator 20are set such that the deformation of seals in the stacking directionunder the load applied in the stacking direction is substantiallyequalized (see FIG. 6).

As shown in FIG. 7, the surface 20 b of second metal separator 20 isprovided with an inlet manifold 72 which establishes fluid communicationbetween coolant inlet communication hole 32 a and coolant flow channels46, and an outlet manifold 74 which establishes fluid communicationbetween coolant outlet communication hole 32 b and coolant flow channels46. The inlet manifold 72 is constructed by a plurality of supports 76which are spaced apart in the direction of arrow C, extend in thedirection of arrow B, and constitute outside and inside seals 58 c and58 d. The outlet manifold 74 is similarly constructed by a plurality ofsupports 78 which are spaced apart in the direction of arrow C, extendin the direction of arrow B, and constitute outside and inside seals 58c and 58 d.

The inlet manifold 72 overlies seal laps 80 a and 80 b constitutingoutside and inside seals 58 a and 58 b on surface 20 a, with secondmetal separator 20 interposed therebetween.

Similarly, supports 78 constituting outlet manifold 74 overlie seal laps82 a and 82 b of outside and inside seals 58 a and 58 b, while being onopposite surfaces 20 a and 20 b of second metal separator 20, as shownin FIG. 7.

As shown in FIG. 7, on the surface 20 b of second metal separator 20, aninlet manifold 84 and an outlet manifold 86 are provided in proximity tofuel gas inlet communication hole 34 a and fuel gas outlet communicationhole 34 b, respectively. The inlet manifold 84 is provided with aplurality of supports 88 arranged in the direction of arrow C, and theoutlet manifold 86 is similarly provided with a plurality of supports 90arranged in the direction of arrow C.

The supports 88 of inlet manifold 84 overlie seal laps 92 a and 92 b ofoutside and inside seals 58 a and 58 b, with second metal separator 20interposed therebetween. Similarly, the supports 90 of outlet manifold86 overlie seal laps 94 a and 94 b of outside and inside seals 58 a and58 b, with second metal separator 20 interposed therebetween.

The inlet manifold 84 and seal laps 92 a, 92 b, and the outlet manifold86 and seal laps 94 a, 94 b are set such that the deformation of sealsin the stacking direction under the load applied in the stackingdirection is substantially equalized. Specifically, the inlet manifold84 is constructed as is the inlet manifold 72. A plurality of feed holes96 and discharge holes 98 are formed in proximity to inlet and outletmanifolds 84 and 86 and disposed outward of inside seal 58 d. The feedholes 96 and discharge holes 98 are formed throughout the separatorinward of inside seal 58 b on the surface 20 a side of second metalseparator 20 and at the inlet and outlet sides of fuel gas flow channels44 (see FIG. 6).

Although outside seal 58 c is formed as a coolant seal on the surface 20b of second metal separator 20 in the illustrated embodiment, theinvention is not limited thereto. Such a coolant seal may be formed onthe surface 18 b of first metal separator 18.

EXAMPLE

Examples and Comparative Examples are given below for furtherillustrating the invention, but the invention is not limited thereto.All parts are by weight.

Example 1

Dimethylvinylsiloxy-endcapped dimethylpolysiloxane #1 having an averagedegree of polymerization of 500, 60 parts, was mixed with 18 parts of aresinous copolymer consisting of (CH₃)₃SiO_(1/2) units,CH₂═CH(CH₃)₂SiO_(1/2) units and SiO₂ units([(CH₃)₃SiO_(1/2)+CH₂═CH(CH₃)₂SiO_(1/2)]/SiO₂=0.95 in molar ratio, vinylcontent=0.00025 mol/g, Mw=2,200), 22 parts of fumed silica having a BETspecific surface area of 200 m²/g (Aerosil 200, Nippon Aerosil Co.,Ltd.), 5 parts of hexamethyldisilazane, and 2.0 parts of water at roomtemperature for 30 minutes. The mixture was heated at 150° C., agitatedat the temperature for 3 hours, and cooled. To this mixture, 1 part ofcarbon powder A (Denka Black HS-100, Denki Kagaku Kogyo K.K., BETspecific surface area=39 m²/g, iodine adsorption value 52 mg/g, DBPabsorption value 140 ml/100 g) was added. The mixture was worked once ona three-roll mill, yielding a silicone rubber base. This silicone rubberbase, 100 parts, was combined with 60 parts of dimethylpolysiloxane #1and milled for 30 minutes. To the mixture were added 3.4 parts (giving[Si—H/alkenyl]=1.5 in molar ratio) of methylhydrogenpolysiloxane #2having Si—H groups at both ends and side chains (degree ofpolymerization 25, Si—H content 0.0048 mol/g) as a crosslinking agentand 0.05 part of ethynyl cyclohexanol as a reaction regulator. Continuedmilling for 15 minutes gave a silicone rubber composition. This siliconerubber composition was combined with 0.1 part of a platinum catalyst (Ptconcentration 1 wt %), press cured at 120° C. for 10 minutes, and postcured in an oven at 200° C. for 4 hours. The cured sample was measuredfor hardness and compression set according to JIS K6249, with theresults shown in Table 1. Note that the compression set was measuredunder two sets of conditions: 120° C.×500 hours in air and 120° C.×500hours in 0.01N sulfuric acid solution.

There were furnished 60 wt % carbon-filled epoxy resin and stainlesssteel (SUS) plates having a primer coated thereon (Primer No. 101A/B,Shin-Etsu Chemical Co., Ltd., air drying+150° C.×30 min baking). Thesilicone rubber composition was press cured to the epoxy resin andstainless steel plates at 150° C. for 5 minutes to form a rubber layerof 0.5 mm thick, and post cured at 200° C. for 4 hours. The samples wereimmersed in 0.01N sulfuric acid solution at 120° C. for 500 hours. Afterremoval, the samples were examined by a bond strength test where apercent cohesive failure was computed by peeling the rubber layer. Theresults are shown in Table 2.

Example 2

Dimethylpolysiloxane #1 (in Example 1), 68 parts, was mixed with 30parts of a resinous copolymer consisting of (CH₃)₃SiO_(1/2) units,CH₂═CH(CH₃)₂SiO_(1/2) units, SiO₂ units and (CH₃)₂SiO_(1/2) units([(CH₃)₃SiO_(1/2)+CH₂═CH(CH₃)₂SiO_(1/2)]/SiO₂=0.72 in molar ratio,(CH₃)₂SiO content 15 wt %, vinyl content=0.00015 mol/g, Mw=8,100), 22parts of fumed silica having a BET specific surface area of 300 m²/g(Aerosil 300, Nippon Aerosil Co., Ltd.), 5 parts ofhexamethyldisilazane, 0.3 part of divinyltetramethyldisilazane, and 2.0parts of water at room temperature for 30 minutes. The mixture washeated at 150° C., agitated at the temperature for 3 hours, and cooled.To this mixture, 2 parts of carbon powder B (Denka Black 100% Press,Denki Kagaku Kogyo K.K., BET specific surface area=65 m²/g, iodineadsorption value 88 mg/g, DBP absorption value 160 ml/100 g) was added.The mixture was worked once on a three-roll mill, yielding a siliconerubber base. This silicone rubber base, 120 parts, was combined with 40parts of dimethylpolysiloxane #3 capped with dimethylvinylsiloxy at bothends, containing vinyl on side chains, and having an average degree ofpolymerization of 420 and milled for 30 minutes. To the mixture wereadded 5.8 parts (giving [Si—H/alkenyl]=2.0 in molar ratio) ofmethylhydrogenpolysiloxane #2 having Si—H groups at both ends and sidechains (degree of polymerization 25, Si—H content 0.0048 mol/g) as acrosslinking agent and 0.05 part of ethynyl cyclohexanol as a reactionregulator. Continued milling for 15 minutes gave a silicone rubbercomposition. This silicone rubber composition was combined with 0.1 partof a platinum catalyst (Pt concentration 1 wt %), after which it wastested for hardness, compression set, and bond strength as in Example 1.The results are shown in Tables 1 and 2.

Comparative Example 1

Dimethylpolysiloxane #1 (in Example 1), 60 parts, was mixed with 18parts of a resinous copolymer consisting of (CH₃)₃SiO_(1/2) units,CH₂═CH(CH₃)₂SiO_(1/2) units and SiO₂ units([(CH₃)₃SiO_(1/2)+CH₂═CH(CH₃)₂SiO_(1/2)]/SiO₂=0.95 in molar ratio, vinylcontent=0.00025 mol/g, Mw=2,200), 22 parts of fumed silica having a BETspecific surface area of 200 m²/g (Aerosil 200, Nippon Aerosil Co.,Ltd.), 5 parts of hexamethyldisilazane, and 2.0 parts of water at roomtemperature for 30 minutes. The mixture was heated at 150° C., agitatedat the temperature for 3 hours, and cooled, yielding a silicone rubberbase. This silicone rubber base, 100 parts, was combined with 60 partsof dimethylpolysiloxane #1 (in Example 1) and milled for 30 minutes. Tothe mixture were added 3.4 parts (giving [Si—H/alkenyl]=1.5 in molarratio) of methylhydrogenpolysiloxane #2 having Si—H groups at both endsand side chains (degree of polymerization 25, Si—H content 0.0048 mol/g)as a crosslinking agent and 0.05 part of ethynyl cyclohexanol as areaction regulator. Continued milling for 15 minutes gave a siliconerubber composition. This silicone rubber composition was combined with0.1 part of a platinum catalyst (Pt concentration 1 wt %), after whichit was tested for hardness, compression set, and bond strength as inExample 1. The results are shown in Tables 1 and 2.

Comparative Example 2

Dimethylpolysiloxane #1 (in Example 1), 68 parts, was mixed with 30parts of a resinous copolymer consisting of (CH₃)₃SiO_(1/2) units,CH₂═CH(CH₃)₂SiO_(1/2) units, SiO₂ units and (CH₃)₂SiO units([(CH₃)₃SiO_(1/2)+CH₂═CH(CH₃)₂SiO_(1/2)]/SiO₂=0.72 in molar ratio,(CH₃)₂SiO content 15 wt %, vinyl content=0.00015 mol/g, Mw=8,100), 22parts of fumed silica having a BET specific surface area of 300 m²/g(Aerosil 300, Nippon Aerosil Co., Ltd.), 5 parts ofhexamethyldisilazane, 0.3 part of divinyltetramethyldisilazane, and 2.0parts of water at room temperature for 30 minutes. The mixture washeated at 150° C., agitated at the temperature for 3 hours, and cooled.To this mixture, 2 parts of carbon powder C (Ketjen Black EC, LionCorp., BET specific surface area=950 m²/g, iodine adsorption value 850mg/g, DBP absorption value 350 ml/100 g) was added. The mixture wasworked once on a three-roll mill, yielding a silicone rubber base. Thissilicone rubber base, 120 parts, was combined with 40 parts ofdimethylpolysiloxane #3 capped with dimethylvinylsiloxy at both ends,containing vinyl on side chains, and having an average degree ofpolymerization of 420 and milled for 30 minutes. To the mixture wereadded 6.6 parts (giving [Si—H/alkenyl]=2.0 in molar ratio) ofmethylhydrogenpolysiloxane #2 having Si—H groups at both ends and sidechains (degree of polymerization 25, Si—H content 0.0048 mol/g) as acrosslinking agent and 0.05 part of ethynyl cyclohexanol as a reactionregulator. Continued milling for 15 minutes gave a silicone rubbercomposition. This silicone rubber composition was combined with 0.1 partof a platinum catalyst (Pt concentration 1 wt %), after which it wastested for hardness, compression set, and bond strength as in Example 1.The results are shown in Tables 1 and 2.

TABLE 1 Comparative Example Example 1 2 1 2 Hardness, Durometer A 42 4841 50 Compression set in air, % 8.8 9.5 11.6 13.5 Compression set in0.01N H₂SO₄, % 14.5 17.9 19.9 26.8

TABLE 2 Cohesive failure Comparative Example Example 1 2 1 2 Stainlesssteel  95% 100% 55% 100% Carbon-filled epoxy resin 100% 100% 25% 100%

It was examined whether an integral seal-separator having the structureshown in FIG. 5 was moldable from the silicone rubber composition. Astainless steel substrate having a primer coated thereon (Primer No.101A/B, Shin-Etsu Chemical Co., Ltd., air drying+150° C.×30 min baking)was furnished as a separator. By insert molding, the silicone rubbercomposition was molded and cured around the substrate in a mold at atemperature of 150° C. for 5 minutes. After removal from the mold, thesample was cured at 200° C. for 4 hours and examined by a bond strengthtest where a percent cohesive failure was computed by peeling the rubberlayer. Provided that samples having a cohesive failure of 100% pass thetest, a percent of defectives is shown in Table 3.

TABLE 3 Moldability Comparative Example Example 1 2 1 2 Defective bond0% 0% 60% 0%

Japanese Patent Application No. 2007-317420 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

The invention claimed is:
 1. A separator seal providing electricinsulation of a volume resistivity of at least 1.0 TΩ·m for use inpolymer electrolyte fuel cells which is formed of a sealing compositionin the cured state, said sealing composition comprising: (A) 100 partsby weight of an organopolysiloxane containing at least two alkenylgroups each attached to a silicon atom in a molecule, saidorganopolysiloxane having the following average compositional formula(I):R¹ _(a)SiO_((4-a)/2)  (I) wherein R¹ which may be the same or differentis a monovalent hydrocarbon group selected from the groups consisting ofalkyl groups, aryl groups, aralkyl groups, alkenyl groups, andsubstituted forms of the foregoing groups in which some or all hydrogenatoms are replaced by halogen atoms or cyano groups, and “a” is apositive number in the range 1.5 to 2.8, (B) 5 to 50 parts by weight ofa resinous copolymer composed mainly of R₃SiO_(1/2) units and SiO₂ unitsin a molar ratio between 0.5:1 and 1.5:1, wherein R is a substituted orunsubstituted monovalent hydrocarbon group, said resinous copolymercontaining 5×10⁻³ to 1×10⁻⁴ mol/g of vinyl groups and having a weightaverage molecular weight of 1000 to 10,000, (C) 0.5 to 20 parts byweight of an organohydrogenpolysiloxane containing at least threehydrogen atoms each attached to a silicon atom in a molecule, (D) 10 to30 parts by weight of fumed silica having a BET specific surface area of50 to 400 m²/g, (E) 0.8 to 1.9 parts by weight of carbon powdercomprising carbon black having a BET specific surface area of 39 to 65m²/g, an iodine adsorption value of 52 to 88 mg/g, and a DBP absorptionvalue of 100 to 200 ml/100 g, and (F) 0.5 to 1,000 ppm of an additionreaction catalyst as platinum group metal based on the weight ofcomponent (A).
 2. The separator seal of claim 1 wherein theorganopolysiloxane (A) has an average degree of polymerization of 100 to2,000, at least 90 mol % of the entire organic groups attached tosilicon atoms being methyl.
 3. The separator seal of claim 1 wherein amolar ratio of Si—H groups in component (C) to total alkenyl groups incomponents (A) and (B) is between 0.8:1 and 5.0:1.
 4. A separator foruse in polymer electrolyte fuel cells comprising a substrate comprisinga metal thin plate or a conductive powder and a binder and a seal formedat a periphery on at least one surface of the substrate, said sealcomprising the separator seal of any one of claims 1 to
 3. 5. Theseparator seal of claim 1, wherein the carbon powder (E) includes atleast one selected from the group consisting of acetylene black,conductive furnace black, super-conductive furnace black,extra-conductive furnace black, conductive channel black, and furnaceblack and channel black which have been heat treated at a temperature onthe order of 1500° C.